The pulp filled cavity in the root of the tooth is known as the root canal. When the pulp and the dentin on the root canal wall becomes infected, they must be removed as the infection causes severe pain for the patient. The procedure for removing the pulp and infected dentin is appropriately designated as a root canal.
The removal of the infected dentin is accomplished by use of a small cylindrical file, usually rotated within the canal manually. In performing this procedure, a skilled dentist is concerned with dual objectives: (1) debriment or cleaning of the canal and (2) shaping of the canal which helps achieve proper insertion of the obturating filling material.
Shaping of the root canal is a difficult task even for a skilled dentist as the root canal has various degrees of spatial curvatures. When shaping in these areas, it is particularly important to prevent breakage of the files within the canal, creating deviations or uneven/unequal canal wall preparations, or ledging the canal curvature. Furthermore, improper filing can lead to over preparation of the outside canal wall with under preparation of the inside canal wall in the three dimensional canal.
Even with all of the foregoing considerations when preparing the canal, most dentists still use a variation of the traditional, manual method of filing. The original manual method involves insertion of a small file into the canal, manually turning the file 1/4 of a turn in the clockwise direction, followed by a withdraw. This method gives the dentist excellent tactile sensation while filing, but it has several major shortcomings. The 1/4 turn is performed by rolling the file between the forefinger and the thumb. Insertion and withdraw is accomplished by wrist movements. A combination of both of these motions leads to fatigue of the dentist's hand. In addition to hand fatigue, the traditional method also does not prevent excess force form being applied against the canal wall and often a file will lock into the wall and break inside the canal. Furthermore, if the file is twisted into an unparallel angle with the canal wall when turned, it will cause unequal/uneven wall preparation which is difficult to obturate.
In an effort to overcome the "human error" associated with the precision filing of a root canal, automated dental tools were introduced. It was hoped that these automated tools would be able to simulate the 1/4 clockwise turn of the traditional method. The device however did not acquire much support from the dental community for three main reasons. First, it rotated at about 3,000 rpm and often created poorly shaped canals due to an almost complete lack of tactile sensation. Second, the file frequently locks into the canal wall, causing it to break. Third, for power, it was attached to a constricting air line.
The failure of this device led to the next generation of automated root canal handpieces, ultrasonic and sonic devices using a purely reciprocal motion. The biggest draw back to these devices was the loss of tactile sense. This led to poor shaping involving elbows and alterations of the canal foramen. The sonic device led to excessive breakage of files and both instruments were restricted by being attached to air lines or special units.
The most recent development was an improvement to the automated rotating dental tool, this one rotating at almost 1/10 of the speed -350 rpms. This device rotates a full 360 degrees and is powered either by air or electricity. Again, this device compromises tactile sensation as it is large, has a restrictive cord or air line attached, and is operated by foot rheostat. The foot control has proved to be an imprecise method of regulating file rotations as evidenced by excessive file breakage.
The major failure of all mechanical handpieces is difficult to overcome as it is attributable to an inherent feature of the file. The file is supposed to cut, therefore, when rotated in the clockwise cutting direction, it has a natural tendency to catch the canal wall, stress, weaken, and then break. This result occurs more quickly if the canal is curved or the file is rotated at excessive speeds.
A variation of the traditional manual technique was implemented to overcome this inherent tendency of the file in treating the canal wall. This technique involves twisting the file in alternating clockwise and counter clockwise rotations between the forefinger and thumb when the file is inserted inside the canal. The clockwise motion sets and locks the file into the canal wall while the counter clockwise motion unthreads the file and balances the stress deformation. The major problems with this method is again breakage, due to the imprecise force applied by the dentist in the clockwise direction, and human hand fatigue and stretching of the ligaments between the thumb and forefinger of the dentist.
What is needed to overcome and improve this concept is a device which automates the counter clockwise rotation while still maintaining the clockwise insertion but in a less forceful manner to prevent file hang up and breakage and give the dentist the tactile sensation necessary to skillfully prepare the delicate root canal wall.